Several types of equipment are used downhole in oil and gas wells, or beneath the surface of the sea adjacent to offshore wells. These types of equipment may be electrically operated or actuated. Examples of such equipment include certain wireline tools and remote well telemetry equipment. The electrical power required can be provided by connecting the device to a surface power source via electrical cables, or by placing a power source near the site of the device itself. Often it is not practical to use electrical cables running from the surface to the subterranean or subsea site of the electrically-powered device, because of the great distance involved, or because the cables can interfere with the passage of other equipment through the wellbore, and may be vulnerable to being damaged during well operations. In particular, during logging while drilling, where the drill pipe rotates, a cable can not typically be run either for telemetry or for power.
Applications in logging while drilling or measurement while drilling (collectively known as LWD) often require power. Typically power in LWD applications is supplied by a turbine through which drilling mud is circulated. Power output of about 200 W is common from the turbine. However, when drilling mud is not being circulated, auxiliary power may be required. Auxiliary power may be provided by a high temperature lithium thionyl chloride battery, for example. In small configurations, these batteries may achieve an energy density of 500 Whl−1. While these batteries have a nominal temperature limit of 150° C., ratings as high as 200° C. may be accomplished through specific design. However, using these batteries presents another host of obstacles. For instance, these batteries tend to be expensive, and because of the inherently reactive nature of thionyl chloride, transportation and disposal restrictions apply. Costs are often increased because downhole operational procedures may require that these batteries be disposed of even after partial use, because every LWD job may require a fresh power pack. In addition, increasing the power and energy generation capacity of a battery generally requires a proportionate increase in the size of the battery, which can present difficulties given the limited space constraints that exist in wellbores. These batteries are not rechargeable, thereby requiring batteries with sufficient energy capacity to last the duration of a logging operation.
Fuel cells may be used as a local source of power for downhole and subsea electrical devices. Fuel cells make use of an electrochemical reaction involving a fuel and an oxidant in a cell that comprises an anode, cathode, catalyst, gas distribution layers and electrolyte, to generate electricity without generating the unwanted by-products associated with combustion, while providing relatively higher energy efficiency. Fuel cells potentially have a number of advantages over other power generation or storage means in many applications. In a fuel cell, water is electrochemically synthesized from its elemental constituents, hydrogen and oxygen. The useful part of the heat of formation for this reaction, Hf, is the Gibbs free energy, Gf. In a fuel cell, the Gibbs free energy is converted to electrical work and heat.
A number of obstacles have hindered the use of fuel cells in downhole and subsea applications. For example, in a continuous operation, a steady supply of the reactant gases, and removal of the produced water is necessary, not easily achieved in oilfield downhole applications. Continuous operation is also restricted by the amount of gases carried in the containers and the associated weight constraints. Furthermore, water disposal requires pumping to a borehole at high pressure, which creates its own difficulty. Fuel cells typically include one or more pumps to provide circulation of fuel and/or oxidant in a closed loop through the cell. If such a pump fails downhole, repair or replacement can be extremely expensive, given the need to retrieve the fuel cell to the surface. Further, the operation of the pumps consumes some of the energy produced by the cell, thus reducing the net power yield available to operate an external device. This latter point can be a significant problem in downhole or subsea applications in which a supply of power is needed for certain period of time, and yet space constraints limit the ability to simply increase the size of the fuel and oxidant reservoirs. Additionally, the reaction product, such as liquid water or water vapor, needs to be removed from the fuel cell stack in order to continuously run the fuel cell. Removal of the water in a downhole presents a challenge because the surrounding pressure is higher than that present in a conventional fuel cell placed at surface in an ambient environment and operating in air. Using a pump to expel the water into the high pressure downhole or may require a large amount of power making such a system impractical.
There is a need for a new power supply apparatus that can provide the electrical power needed to operate various downhole equipment. Such a power supply needs to be compact and rugged for a downhole environment. Additionally such a power supply apparatus needs to be able to supply power when drilling mud is not being circulated in an LWD environment.